Electrical primary control system for furnaces

ABSTRACT

An electrical primary control system for furnaces and the like including burner control means, means including solid state means effective to control said burner control means in response to a flame detector signal, line voltage safety switch means, and means for interfacing between an isolated low voltage control circuit and said burner control means.

United States Patent Bauer July 1, 1975 ELECTRICAL PRIMARY CONTROL 3,624,407 11/1971 Bauer 307/116 SYSTEM FOR FURNACES 3,732,433 5/1973 Lourigan 307/117 Inventor: Frederick T. Bauer, Holland, Mich.

Robertshaw Controls Company, Richmond, Va.

Dec. 4, 1973 Assignee:

Filed:

Appl. No.:

US. Cl. 307/117; 337/361; 431/78 Int. Cl. l-lOlh 37/00 Field of Search 307/116, 117', 431/78;

References Cited UNITED STATES PATENTS 4/1963 Boudouris 337/72 X Primary Examiner-Robert K. Schaefer Assistant ExaminerM. Ginsburg Attorney, Agent, or Firm-Malcolm R. McKinnon 5 7 ABSTRACT 10 Claims, 5 Drawing Figures g 2m i0 74' if Jj 1/) i// in z/z 4/ I I/ Mr 1d m /J H l x/n C I I l W 2/ i7 a j 1 12' i;

ELECTRICAL PRIMARY CONTROL SYSTEM FOR FURNACES BRIEF SUMMARY OF THE INVENTION This invention relates to electrical primary control systems for furnaces and the like and, more particularly, to an improved electrical primary control system incorporating improved control circuitry and improved line voltage safety switch means and effective to control and supervise a furnace burner.

Heretofore, primary controls have been utilized to control and supervise burners in furnaces, such primary controls controlling the furnace burner in response to a low voltage separate thermostat, usually located in the living space of a dwelling or other building, and supervising the furnace burner to insure safe combustion in the furnaces combustion chamber and shutting the burner off if an unsafe condition occurs.

An object of the present invention is to provide an improved primary control system for furnaces incorporating improved control circuitry which provides improved furnace burner control and supervision.

Another object of the invention is to provide an improved primary control system for furnaces incorporating improved line voltage safety switch means.

Another object of the invention is to provide an improved primary control system for furnaces operable to control a furnace burner in response to a low voltage thermostat and to de-energize the burner in the event an unsafe condition occurs.

Another object of the invention is to provide an improved primary control system for furnaces that is extremely durable, efficient and reliable in operation and readily adaptable to meet the control and supervision requirements of various types of furnaces.

The above as well as other objects and advantages of the present invention will become apparent from the following description, the appended claims and the accompanying drawings.

BREIF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic circuit diagram of an electrical primary control system embodying the present invention;

FIG. 2 is a cross-sectional view of a line voltage safety switch incorporated in the system illustrated in FIG. 1, showing the same in the closed condition;

FIG. 3 is a view similar to FIG. 2 and illustrating the switch in the open condition;

FIG. 4 is a fragmentary, cross-sectional view of the structure illustrated in FIG. 2, taken on the line 44 thereof; and

FIG. 5 is a view similar to FIG. 2 and illustrating the open switch while an attempt is being made to reset the same.

DETAILED DESCRIPTION Referring to the drawings, and more particularly to FIG. 1 thereof, the circuitry for an electrical primary control system, generally designated 10, embodying the present invention is schematically illustrated therein. As shown in FIG. 1, the primary control system is comprised of a step down transformer 12 having a primary winding 14 and secondary windings l6 and 18, the primary winding 14 being adapted to be connected to a conventional source of I volt alternating current while, in the embodiment of the invention illustrated,

each of the secondary windings l6 and 18 of the isolated stepdown transformer preferably has a potential of approximately 8 volts AC. The primary control 10 also includes a line voltage safety switch, generally designated 20, including normally closed contacts 21 and 22 and a heater coil 23; a conventional thermostat generally designated 24; a reed switch, generally designated 25, having contacts 26 and 28 and independent, concentrically wound coils RC 1 and RC2, the contacts 26 and 28 being enclosed within a hermetically sealed glass envelope 30 while the coils RC1 and RC2 are concentrically wound therearound; a triac Q1 and a silicon controlled rectifier SCRl. The primary control system 10 also includes a cadmium sulfide flame detector 32, resistors R1, R2, R3, R4, R5, R6, R8, R9, and R10; capacitors, C2 and C3; a potentiometer R7 and diodes D1 and D2. As shown in FIG. 1, the primary control system 10 is connected to and adapted to control and supervise a conventional burner 34 ofa furnace (not shown). The terminal 35 of the burner 34 is connected to the source of power by the lead Ll while the terminal 36 of the burner is connected to the terminal 37 of the triac Ql by the lead L2, the terminal 38 of the triac Q1 being connected to the source of power by the lead L3. The contact 26 of the reed switch 25 is connected by the lead L4 to the lead L2 while the contact 28 is connected to the gate 39 of the triac 01 through the resistor R8 by the lead L5, the resistor R9 and capacitor C3 being connected across the leads L2 and L3 by the leads L4 and L6 to protect the triac Q1.

The normally closed contacts 21 and 22 of the safety switch 20 are connected to and adapted to make and break the high voltage lead 120 connected to the primary winding 14 of the transformer 12.

The terminal 40 of the secondary windings 16 is connected to the heater coil 23 of the safety switch 20 and to the terminal 42 of the thermostat 24 by the leads L7 and L8, the terminal 44 of the thermostat 24 being connected by the lead L9 through the resistors R5 and R6 and the coil RC1, to the terminal 46 of the diode D1. The terminal 48 of the diode D1 is connected to the center tap 50 of the secondary windings of the transformer 12 by the lead L10. The capacitor C2 is connected across the resistor R6 and the coil RC1 of the reed switch 25 by the lead L11 while the resistor R3 is connected between the leads L9 and L10 by the lead L12 as illustrated in FIG. 1. The terminal 44 of the thermostat 24 is also connected to the terminal 52 of the diode D2 by the lead L13 through the resistor R4, the terminal 54 of the diode D2 being connected to the center tap 50 of the transformer 12 by the lead L10. As shown in FIG. 1, the heater coil 23 of the safety switch 20 is connected to the terminal 56 of the silicon controlled rectifier SCRl through the coil RC2 of the reed switch 25 of the lead L17, the potentiometer R7 being connected across the coil RC2. The terminal 58 of the silicon controlled rectifier SCRl is connected to the center tap of the transformer 12 by the leads L14 and L10 while the gate 60 of the rectifier SCRl is connected to the terminal 52 of the diode D2 by the lead L15 and to the terminal 62 of the secondary winding 18 of the transformer by the lead L16 through the resistor R1, the cadmium sulfide flame detector 32 and the resistor R2, the resistor R10 being connected across the flame detector 32 and functioning to stabilize the silicon controlled rectifier SCRI.

The above described components are preferably mounted on a circuit board which may be integrated, and mounted within a housing (not shown).

Referring in greater detail to the safety switch 20 illustrated structurally in FIGS. 2, 3, 4 and 5, the safety switch 20 is comprised of a housing means 71 carrying a pair of spring switch blades 72 and 73 respectively interconnected at the lower ends thereof to terminal means 74 and 75 that project from the housing means 71 and carry at the upper ends thereof the contacts 21 and 22 that are adapted to be placed in electrical contact with each other in the manner illustrated in FIG. 2 to complete an electrical circuit therebetween. However, the normal bias of such switch blades 72 and 73 is to tend to hold the contacts 21 and 22 out of electrical contact with each other in the manner illustrated in FIG. 3 so as to open the electrical circuit therebetween and break the high voltage line 120.

The electrical switch 20 is constructed and arranged in a manner such that should the system detect that the burner 34 has not been ignited within a predetermined time period, the system 10 will cause the electrical switch to be tripped to the open condition illustrated in FIG. 3 and thereby break the high voltage line 120 until the switch 20 is manually reset from the open or tripped condition illustrated in FIG. 3 back to the closed position illustrated in FIG. 2 in a manner hereinafter described.

A bimetallic latch member 81 is provided which is disposed in a chamber 80 formed by the housing means 71 and has an L-shaped end 82 interconnected to a flexure member 83 that is fixed to the housing means 71 by a fixed support 84, the other end 85 of the bimetallic latch member 81 being configured in such a manner that the same is adapted to engage against an angled surface 86 of a first cam 87 rotatably mounted to the housing means 71 by a shaft 88 carried by the hous ing means 71.

The cam 87 is so constructed and arranged that the same has a lobe 89 adapted to engage against the spring blade 72 and bias the same to the right in the manner illustrated in FIG. 2 when the cam member 87 is disposed in the position illustrated in FIG. 2 so as to maintain the contact 21 of the blade 72 in full electrical contact with the contact 22 of the blade 73 as such position of the cam 87 not only biases the blade 72 to the right, but such a position of the cam 87 also biases the blade 73 to the right so that the blade 73 is always spring biased against the contact 21 of the blade 72 when the cam 87 is in the latched position illustrated in FIG. 2. Because the spring blades 72 and 73 are tending to rotate the cam 87 in a clockwise direction as illustrated in FIG. 2, the abutment end 85 of the latch 81 prevents such movement of the cam 87 so that the cam 87 is latched in the position illustrated in FIG. 2 thereby maintaining an electrical circuit between the switch blades 72 and 73 for the purpose previously described.

However. the bimetallic latch member 81 is adapted to be warped to the left to move the latching end 85 thereof out of the path of the surface 86 of the cam 87 when the bimetallic member 81 is heated to a predetermined extent by the electrical heater coil 23 that is carried by the bimetallic latch member 81 and has one end 91 thereof electrically interconnected by a lead 92 to a terminal 93. The other end 94 of the electrical heater 23 is interconnected by a lead 95 to a terminal 96, the

terminals 93 and 96 being connected to the leads L8 and L17, respectively.

As will be described hereinafter in greater detail, when the system 10 is initially energized by the thermostat 24 calling for heat, electrical current is adapted to flow through the electrical heater coil 23 whereby the electrical heater coil 23 begins to heat up the latch member 81 to tend to bend the same to the left and thereby open the electrical circuit being provided by and through the switch blades 72 and 73. However, if the burner 34 ignites during such time period, the detector 32 senses the flame at the burner 34 and the system de-energizes the electrical heater coil 23 whereby the latch member 81 will remain in its latched position as illustrated in FIG. 2 to maintain an electrical circuit through the switch blades 72 and 73 and thereby maintain the high voltage 120 closed.

However, if during such predetermined time period the burner 34 is not ignited, the heater coil 23 will remain energized whereby the bimetallic member 81 is heated sufficiently and thereby warped to the left to pull the latch end thereof away from the surface 86 of the cam 87 whereby the natural bias of the switch blade 72 rotates the cam 87 from the position illustrated in FIG. 2 to the position illustrated in FIG. 3 so that the natural bias of the switch blades 72 and 73 holds the blades 72 and 73 in an open condition thereof to open the contacts 21 and 22 and break the high voltage line 120 to terminate the flow of current to the burner 34 and, thus, turn off the burner until the electrical switch 20 is reset in a manner hereinafter described.

An ambient temperature compensating bimetal member 102 has a lower L-shaped end 103 thereof also interconnected to the flexure 83 by the same fastening member 84 that fastens the L-shaped end 82 of the bimetal latch member 81 thereto as illustrated. The upper end 105 of the bimetal member 102 has a suitable opening (not shown) passing therethrough and receiving a reduced part 106 of a threaded adjusting member 107 therein, the threaded adjusting member 107 being threaded in a threaded bore (not shown) passing through a nut 108 carried in suitable recess means 109 in the housing means 71.

The reduced portion 106 of the threaded adjusting member 107 carries a disc-like member 110 on the outer end thereof and a washer-like member 111 is loosely telescoped onto the reduced part 106 of the adjusting member 107, the member 110 being urged to the right as illustrated in FIG. 2 toward the disc 110 by a compression spring 112 disposed between the slideable disc 111 and the nut 108 as illustrated. In this manner, the end 105 of the compensating bimetal member 102 is held against the disc 110 of the adjusting mem her 107 by the disc 11 under the force of the compression spring 112.

In this manner, rotation of the adjusting member 107 at the bifurcated end 113 thereof that projects out of the housing means 71 at an opening 114 thereof will cause axial movement of the adjusting member 107 relative to the housing means 71 and thereby adjust the position of the upper end 105 of the compensating bimetal member 102 which, through its lower end 103, will change the position of the flexure 83 and, thus, the position of the latching bimetal member 81 so that the time period required for the heater means 23 to be operating before the bimetal member 81 will move to its unlatching position relative to the cam 87 can be adjusted within the limits of the adjusting member 107.

In particular, when the threaded adjusting member 107 is threaded into the housing means 71 the flexure 83 is flexed to the right as illustrated in FIG. 2 and thereby requires a longer time period for the bimetal member 81 to be heated by the heater coil 23 before the same will move to an unlatching position thereof. Conversely, when the threaded adjusting member 107 is rotated in a manner to move to the left as illustrated in FIG. 2, the flexure 83 is likewise moved to the left and thereby requires a lesser time period for the heater coil 23 to heat the bimetal member 81 to cause the same to move to an unlatching position thereof.

Similarly, the ambient temperature compensating bimetal member 102 is so constructed and arranged that as the same is heated by an increase in the temperature in the atmosphere, the same warps in a direction that tends to move the flexure structure 83 to the right as illustrated in FIG. 2 to offset the likewise heating of the bimetal member 81 by the ambient temperature that tends to warp the bimetal member 81 to its unlatching position. Thus, the bimetal member 102 compensates for such increased ambient temperature. Conversely, a drop in ambient temperature tends to cause the bimetal member 102 to move to the left and thereby carry the flexure 83 to the left an equal amount to the amount that the decreased ambient temperature caused the bimetal member 81 to move to the right and thereby offset that additional amount of time that the heater coil 23 would have to heat the bimetal member 81 to overcome the decreased ambient temperature.

Therefore, it can be seen that the bimetal member 81 is ambient compensated by the bimetal member 102 and can be adjusted within limits by the adjusting member 107 so as to provide the desired time delay that is required for the heater coil 23 to be operated before the bimetal member 81 will move to its unlatching position.

An actuator or reset button 115 projects out through an opening 116 formed through an outer surrounding part 120 of the housing means 71 and is carried on an end 117 of a spring member 118 that has its other end 119 fastened to the part 120 of the housing means 71. The bias of the spring member 118 is such that the same tends to maintain the spring member 118 against the undersurface 121 of the housing part 120' whereby a turned end 122 of the spring member 118 merely projects into an opening 123 formed in the housing means 71 and leading to the chamber 80 thereof. However, when the button 115 is moved downwardly by an operator pushing inwardly on the same, the end 122 of the spring member 118 is moved into the chamber 80 of the housing means 71 and engages against the cam 87 at the surface 86 thereof to pivot the cam 87 in a counterclockwise direction about the shaft means 88 so that the lobe abutment 89 thereof that engages against the spring blade 72 will bow the blade 72 to the right in the manner illustrated in FIG. 5 in an attempt to reset the switch construction should the latch member 81 be in a resetting or relatching position thereof.

Another cam member 124 is also rotatably mounted to the housing means 71 by the shaft means 88 in sideby-side relation to the cam 87 and has an abutment 125 for engaging against the other spring blade 73 in the manner illustrated in the drawings, the cam member 124 being adapted to project through a cut out 126 in the spring blade 72 in the manner illustrated in FIG. 4.

The cam member 124 has a surface 127 that is adapted to be simultaneously engaged by the end 122 of the reset spring member 118 as the same is engaging the cam 87 when the end 122 is moved downwardly by the push button 115 in the manner illustrated in FIG. 5 whereby such movement of the actuator 115 from the unactuated condition thereof of FIG. 3 to the actuated condition thereof of FIG. 5 causes the cam members 87 and 124 to be simultaneously engaged and rotate in unison in a counterclockwise direction so that then ends 89 and 12S thereof will simultaneously move the switch blades 72 and 73 to the right while maintaining the same out of electrical contact with each other in the manner illustrated in FIG. 5. During such movement, if the latch end of the bimetal member 81 is in the path of counterclockwise movement of the cam 87, the cam 87 merely cams by the same to the right to the position illustrated in FIG. 5. Upon release of push button from the actuated position illustrated in FIG. 5, both cam members 87 and 124 tend to rotate back in a clockwise direction under the natural bias of the respective switch blades 72 and 73 back to the natural condition illustrated in FIG. 3. However, if the latch member 81 has its end 85 disposed in the path of movement of the surface 86 of the cam member 87, the same will prevent the cam member 87 from being driven back in a clockwise direction so that the same will hold the switch blade 73 in its biased condition illustrated in FIG. 2 so that clockwise movement of the cam 124 will permit the switch blade 73 to move into electrical contact with the switch blade 72 in the manner illustrated in FIG. 2.

Thus, assuming that the switch 20 is in the latched condition illustrated in FIG. 2 wherein the cam member 87 is held by the end 85 of the latch member 81 from being rotated in a clockwise direction by the natural bias of the spring blade 72, the spring blades 72 and 73 are electrically interconnected together at the contact means 21 and 22 thereof and an electrical circuit is adapted to be provided through the high voltage line 120. When the thermostat calls for heat, the bimetal member 81 remains in its latched condition for the predetermined amount of time as determined by setting the adjusting member in the manner previously described.

During such time period as set by the adjusting member 107, if the burner 34 ignites the fuel, the sensor 32 senses the flames at the burner and the latching bimetal member 81 will remain in the condition illustrated in FIG. 2 and the switch 20 will remain in its latched condition.

However, should the sensor 32 not sense flame at the burner 34 after the lapse of the time period as set by the adjustment member 107, the heater coil 23 will cause the bimetal member 81 to warp to the left and thereby move the end 85 thereof away from the surface 86 of the cam member 87 whereby the earns 87 and 124 are driven back in a clockwise direction by the natural bias of the spring blades 72 and 73 to the position illustrated in FIG. 3 to break the high voltage line 120.

Thereafter, should a person desire to reset the switch 20, that person must push inwardly on the actuator 115 from the unactuated condition thereof illustrated in FIG. 3 to the condition illustrated in FIG. 5 whereby the end 122 of the spring member 118 will act simultaneously against the surfaces 86 and 127 of the cam members 87 and 124 to rotate the same in a counterclockwise direction from the position illustrated in FIG. 3 back to the position illustrated in FIG. 5. Such move ment of the cams 87 and 124 maintain the spring blades 72 and 73 out of contact with each other during such actuated movement of the actuator 115.

A subsequent release of the actuator 115 when in its actuated condition illustrated in FIG, 5 will permit the cams 87 and 124 to be rotated back in a clockwise direction by the natural bias of the spring blades 72 and 73 if the bimetal member 81 has not cooled sufficiently to place the end 85 thereof in the path of movement of the surface 86 of the cam 87. lfthe bimetal member 81 has cooled sufficiently, the end 85 prevents the clockwise movement of the cam 87 so that the same remains in the position illustrated in FIG. 2 and the natural bias of the spring blade 73 causes the cam 124 to move in a clockwise direction and thereby place the contact 22 in electrical contact with the contact 21 of the blade 72 in the manner illustrated in FIG. 2 whereby the switch has been reset to its "on" condition so that the con' trol system can now be operated in the manner previously described.

Thus, it can be seen that the electrical switch 20 is substantially trip free because movement of the reset member 115 to its actuated condition will not permit the switch blades 72 and 73 to be held in a closed position thereof because the switch blades 72 and 73 will only be moved to a closed condition when the actuator 115 is released and if the latching member 81 is in a latched position thereof.

The rectifier SCRl is a conventional silicon controlled rectifier and may, for example, carry a rating of approximately four amperes. The thermostat 24 may be of any desired or conventional construction while the reed switch 25 is preferably of the type comprised of the pair of contacts 26 and 28 carried by reeds hermetically sealed within a glass envelope 30. The reed switch also includes the electrically insulated, independently wound concentric coils RC1 and RC2, the magnetic fluxes of such coils being additive when in phase. The reed switch 25 preferably has a very large differential between pull-in and drop-out ampere turns or coil power. By way of example, the reeds preferably will pull in at about 60 ampere turns, but will not drop out until below 20 ampere turns, a ratio of approximately 3 to I. In the embodiment of the invention illustrated in FIG. 1 the maximum power to the coil RC] is well below that required to pull-in the reed switch and close the contact 26 and 28. The power is, however, enough to hold the reed switch contact 26 and 28 closed once pull-in has been established, due to the very large dif ferential. The reed switch coil RC2, on the other hand, has sufficient power when combined with RC1 to pull in the reed switch. Since the reed switches are very fast they are capable of following an alternating current voltage to open or close 60 or 120 times per second. To avoid this opening and closing and the associated wear, the diode D! and the capacitor C2 are provided. The diode D1 is preferably a 200 milliampere diode which supplies half wave rectified current to the capacitor C2 to establish a DC supply for the reed switch coil RC1. The capacitor C2 is preferably a 47 microfarad 15 volt DC capacitor. The diode D1 and the capacitor C2 function to form a DC supply for the holding coil RC1 so that flux is always present on the coil RC1 when the thermostat calls for heat. This flux is very small however. With such a construction and since relatively small current passes through the contacts 26 and 28, such contacts are very reliable over a relatively long life.

The triac O1 is a bidirectional thyrister which may be gate triggered from a blocking to conducting state for either polarity of applied voltage, and is preferably mounted to isolate the other components of the control 10 from the heat generated by the triac Q1. The resistors RI and R2 are preferably carbon resistors having ratings of ohms and 560 ohms, respectively, one half watt, the purpose of the resistor R1 being to prevent the accidental destruction of the diode D1, transformer 12 or silicon controlled rectifier SCRl by a serviceman in the field. In this connection the resistors R1, R2, R5, R9 and R10, the diode D2 and the capacitor C3 are all provided in the primary control 10 for the purpose of protecting other components and to protect against erroneous wiring in the field. The resistors R1, R2, R5, R9 and R10, the diode D2 and the capacitor C3 are thus not essential to the bisic circuit performance.

Typical values for the components in the control system described above are as follows:

SCRl 4 AMP Silicon controlled rectifier D1 200 Ma diode D2 200 Ma diode Rl Carbon resistor I50 ohms, 1 207, A watt R2 Carbon resistor 560 ohms, 20%, A watt R3 Wircwound resistor 20 ohms. i 20%, 5 watt R4 Carbon resistor 3300 ohms, I 20%, 1% watt R5 Carbon resistor 47 ohms. i 209 A watt R6 Wircwound resistor 680 ohms, 1 2071. 1 watt R7 Wircwound potentiometer l ohm. 20%. 2 watt RB Carbon resistor 82 ohms, i 2071, A watt R9 Carbon resistor 82 ohms. 1 2071, A watt Rlt) Carbon resistor 33000 ohms, 20%, b watt C2 Capacitor 47 mfd 15 VDC C3 Capacitor 22 mfd 200 V Mylar foil It will be understood, however, that these values may be varied depending upon the particular application of the principles of the present invention.

Assuming a basic knowledge of the triac Q1, the silicon controlled rectifier SCRl, and the cadmium sulfide flame detector 32, a typical thermostat cycle operates in the following manner. It should be noted initially that whenever the reed contacts 26 and 28,are closed, current will flow from the source of electric power through the lead Ll, the burner 34, the lead L2, the contacts 26 and 28 and the resistor R8, to the gate of the triac Q1 and the lead L3. When the gate of the triac Q] is energized the full motor current will then pass through the triac O]. This starts the burner and has the same effect as closing a set of relay contacts between the lead L2 and the lead L3.

Whenever the thermostat contacts close, a continuous holding flux is established in the coil RC1 by the DC supply network comprised of the diode D1 and the capacitor C2. Current also flows through the resistor R4 to the gate 60 of the silicon controlled rectifier SCRl. If the cadmium sulfide flame detector 32 registers darkness, no current can be shunted away from the gate 60 of the silicon controlled rectifier SCRl and SCRl will conduct. When SCRl conducts, current also passes through the pull-in coil RC2 of the reed switch 25 and the heater 23 of the safety switch 20. With a flux established in the coil RC2 and the coil RC1, the reed switch contacts 26 and 28 will pull in and the triac Q1 will start the burner. If the cadmium sulfide flame detector 32 does not register flame, the silicon controlled rectifier SCRl will continue to conduct and the safety switch 20 will open the contacts 21 and 22 due to the heating action of the heater 23 effecting the movement of the bimetallic blade 81 as previously described. It is preferred that the contacts 21 and 22 open and lock out after approximately seconds. lf the cadmium sulfide flame detector registers flame, then the flame detector 32 decrease in resistance and shunts current away from the gate 60 of the rectifier SCR1. SCRl will no longer conduct, the heating coil 23 of the safety switch will be deenergized but the coil RC1 will continue to hold in the reed relay contacts 26 and 28. 1f the cadmium cell 32 registers flame and for some reason the flame should go out during the thermostat cycle, the rectifier SCRl will again conduct and the heating coil 23 will be energized so as to open the contacts 21 and 22 into a lock-out condition. When the thermostatic conditions are satisfied and the contacts thereof open, the coil RC1 is deenergized thereby opening the contacts 26 and 28 and also deenergizing the triac Q1. No current is then available through the resistor R4 to energize SCR1 even though the cadmium cell 32 registers no flame. It should also be understood that the same cycle would occur of the thermostat were connected to line voltage and placed in one leg of the transformer primary coil.

An important aspect of the present invention resides in the fact that if there is a failure in the primary control 10, the primary control 10 will fail in safe condition. For example, if the silicon controlled rectifier SCRl is shorted from anode to cathode it will conduct electric current supplied by the secondary winding 16 of the transformer. The cadmium sulfide flame detector 32 will have no eflect on the control circuit. Since current through the rectifier SCRl must also pass through the safety switch heater 23, the safety switch contacts 21 and 22 will open into a lock-out condition. The only way to start the burner again is by depressing the manual reset plunger 115. An open circuit in the rectifier SCRl will render the control circuit inoperative since no starting current is provided in the coil RC2. The burner will thus never start. A short circuit from the gate to the cathode of the rectifier SCRl has the same effect as an open circuit between the anode and cathode of SCRl. An open circuit from the gate to cathode of the rectifier SCRl also has this effect.

Failure of the diode D1 in the short circuit state causes AC voltage to appear across the capacitor C2 and since AC voltage is destructive to the capacitor C2 it will generally cause it to fail short circuited. Hence, there is no coil power to the reed switch coil RC1 and the reed switch is incapable of holding. The burner would then become inoperative. If the diode D1 fails open circuited, there is likewise no power to the coil RC1 and the burner becomes inoperative.

A short circuit failure of the diode D2 reacts the same as a gate to cathode short of the rectifier SCRl as previously described. An open circuit failure of the diode D2 will generally be destructive to the rectifier SCRl and any failure of the SCRl will render the control circuit inoperative as previously described.

An open or short circuit failure of the capacitor C2 will prevent the reed switch 25 from pulling in and the burner from operating. The burner will also be prevented from operating of either of the coils RC1 or RC2 of the reed switch 25 become open or short circuited since such failure will prevent the reed switch from pulling in and closing the contacts 26 and 28.

The resistor R1 prevents accidental destruction of the diode D2 by a serviceman in the field. This could happen if a serviceman accidentally shorted one of the thermostat terminals with the proper terminal of the cadmium sulfide flame detector 32. Open circuit failure would react in the same manner as an open circuit in the flame detector 32. Short circuit of either of the resistors R1 or R2 would simply eliminate the protection measure from the equipment.

The resistor R3 is a wire wound type so that short circuit failure can be neglected. Open circuit failure of the resistor R3 would result in elimination of thermostat bias current used for conventional thermostat preheaters. The resistor R3 plays no role in the circuit other than for this home comfort feature.

Continuing the description of the fail-safe operation of the primary control 10, the resistor R4 is utilized for the purpose of calibrating the cadmium sulfide flame detector 32. If the resistor R4 is open circuited then SCRl never receives current from gate to cathode and will never turn on. Since the rectifier SCRl must conduct to pull in the reed switch through the coil RC2, the burner will never turn on. If the burner is in the middle of a cycle when the resistor R4 fails open, then the burner will fail to start on the next cycle. If the resistor R4 fails in a short circuit condition, then neither of the coils RC1 or RC2 will be energized and the reed switch contacts will not close so that the burner will be inoperative.

The resistor R5 protects the diode D1 from current surges to the capacitor C2 during normal operation. If the resistor R5 were to short circuit then the diode D1 may fail shorted and the burner would become permanently inoperative in the manner previously described in connection with failure of the D1. If the resistor R5 fails open circuited, then no power will be furnished to the coil RC1 and the reed switch contacts will not close. The burner would then be inoperative.

The resistor R6 functions to limit the power to the coil RC1. The resistor R6 is calibrated and calibrates the coil RC1 to within a specified drop-out range for the reed switch. As is well known, wire wound resistors do not fail short. If open circuit failure results, then no power is supplied to the coil RC1 and the reed switch will not pull in. The burner will thus be inoperative if the resistor R6 fails open circuited.

The wire wound potentiometer R7 is used to calibrate the pull-in voltage of the reed switch. This is accomplished by shunting current away from the reed switch coil RC2. An open circuit in the potentiometer R7 allows the reed switch to pull in at lower line voltage than the set-point voltage, as for example volts. Short circuit of the potentiometer R7 prevents power from flowing to the reed switch coil RC2 and the reed switch will not close the contacts 26 and 28. The burner will then be inoperative.

The heating coil of the safety switch 20 cannot fail shorted. An open circuit failure functions in the same manner as an open circuit failure of the anode to cathode on the rectifier SCRl previously described. With respect to the cadmium sulfide flame detector 32 protected by the resistor R10, this flame detector maintains approximately 1500 ohms at one foot candle illumination. Short circuit results in the failure to start the burner when the thermostat closes. An open circuit causes the safety switch 20 to lock out.

While a preferred embodiment of the invention has been illustrated and described, it will be understood that various changes and modifications may be made without departing from the spirit of the invention.

What is claimed is:

1. in an electrical primary control system for furnaces, the combination including burner control means adapted to be connected to a furnace burner connected to a main line source of AC current, a low voltage control circuit including burner ignition detection means and energy conversion means, means providing a substantially lower voltage than line voltage in said control circuit, means in said low voltage control circuit including solid state means effective to actuate said burner control means in response to a signal from said ignition detection means, electrical switch means controlled by said energy conversion means and effective to interrupt the flow of current through said main line source of AC current, and means including a reed switch interfacing between said low voltage control circuit and said burner control means, said electrical switch means comprising a housing means, a pair of movable spring switch blades carried by said housing means and respectively having contact means normally placed out of contact with each other by the normal bias of said switch blades, a pair of cams rotatably carried by said housing means and being respectively engageable with said switch blades, one of said cams when in one position relative to said housing means being adapted to hold its respective switch blade into electrical contact with the other switch blade to complete an electrical circuit therebetween, the other of said cams when in one position thereof relative to said housing means being adapted to hold its respective switch blade out of contact with the other switch blade even though said one cam is in its said one position thereof to thereby prevent completion of said electrical circuit, a movable actuator means including an actuator carried by said housing means and being adapted to simultaneously en gage said cams and move the same to said respective one positions thereof, and latch means for holding said one cam in said one position thereof.

2. The combination as set forth in claim 1 wherein trip means including said energy conversion means is carried by said housing means to move said latch means from its latching position with said one cam whereby the natural bias of said switch blades moves said switch blades to an open condition thereof to open said cir- 12 cuit.

3. The combination as set forth in claim 2 wherein said latch means comprises a bimetal member for latching with said one cam when said bimetal member senses one certain temperature and for warping away from said one cam to unlatch therefrom when sensing another certain temperature.

4. The combination as set forth in claim 3 wherein said energy conversion means is carried by said bimetal member.

5. The combination as set forth in claim 1 wherein said energy conversion means is operable to heat said bimetal member, said trip means including means for adjusting the same so that the amount of time said energy conversion means is to be operated before said bimetal member moves to an unlatching position thereof can be selected within certain limits.

6. The combination as set forth in claim 5 wherein said cams are rotatably carried by said housing means in side-by-side relation.

7. The combination as set forth in claim 6 wherein said cams are rotatably mounted to said housing means by the same shaft means.

8. The combination as set forth in claim 7 wherein said actuator must be moved from an unactuated position thereof to an actuated position thereof to move said cams to said one position thereof, said housing means having biasing means tending to always move said actuator from said actuated position thereof to said unactuated position thereof.

9. The combination as set forth in claim 8 wherein said cams are so constructed and arranged that when said switch blades are out of contact with each other by the normal bias thereof said cams have surfaces disposed in aligned relation that are respectively engaged simultaneously by said actuator to be moved in unison by said actuator when said actuator is moved from said unactuated position thereof to said actuated position thereof whereby actuation of said actuator will not cause said switch blades to contact each other unless said latch means holds said one cam in its said one position thereof when said actuator subsequently moves back from said actuated position thereof to said unactuated position thereof.

10. The combination as set forth in claim 9 wherein the switch blade operated by said one cam has a cut-out therethrough through which said other cam projects to engage its respective switch blade. m a: 

1. In an electrical primary control system for furnaces, the combination including burner control means adapted to be connected to a furnace burner connected to a main line source of AC current, a low voltage control circuit including burner ignition detection means and energy conversion means, means providing a substantially lower voltage than line voltage in said control circuit, means in said low voltage control circuit including solid state means effective to actuate said burner control means in response to a signal from said ignition detection means, electrical switch means controlled by said energy conversion means and effective to interrupt the flow of current through said main line source of AC current, and means including a reed switch interfacing between said low voltage control circuit and said burner control means, said electrical switch means comprising a housing means, a pair of movable spring switch blades carried by said housing means and respectively having contact means normally placed out of contact with each other by the normal bias of said switch blades, a pair of cams rotatably carried by said housing means and being respectively engageable with said switch blades, one of said cams when in one position relative to said housing means being adapted to hold its respective switch blade into electrical contact with the other switch blade to complete an electrical circuit therebetween, the other of said cams when in one position thereof relative to said housing means being adapted to hold its respective switch blade out of contact with the other switch blade even though said one cam is in its said one position thereof to thereby prevent completion of said electrical circuit, a movable actuator means including an actuator carried by said housing means and being adapted to simultaneously engage said cams and move the same to said respective one positions thereof, and latch means for holding said one cam in said one position thereof.
 2. The combination as set forth in claim 1 wherein trip means including said energy conversion means is carried by said housing means to move said latch means from its latching position with said one cam whereby the natural bias of said switch blades moves said switch blades to an open condition thereof to open said circuit.
 3. The combination as set forth in claim 2 wherein said latch means comprises a bimetal member for latching with said one cam when said bimetal member senses one certain temperature and for warping away from said one cam to unlatch therefrom when sensing another certain temperature.
 4. The combination as set forth in claim 3 wherein said energy conversion means is carried by said bimetal member.
 5. The combination as set forth in claim 1 wherein said energy conversion means is operable to heat said bimetal member, said trip means including means for adjusting the same so that the amount of time said energy conversion means is to be operated before said bimetal member moves to an unlatching position thereof can be selected within certain limits.
 6. The combination as set forth in claim 5 wherein said cams are rotatably carried by said housing means in side-by-side relation.
 7. The combination as set forth in claim 6 wherein said cams are rotatably mounted to said housing means by the same shaft means.
 8. The combination as set forth in claim 7 wherein said actuator must be moved from an unactuated position thereof to an actuated position thereof to move said cams to said one position thereof, said housing means having biasing means tending to always move said actuator from said actuated position thereof to said unactuated position thereof.
 9. The combination as set forth in claim 8 wherein said cams are so constructed and arranged that when said switch blades are out of contact with each other by the normal bias thereof said cams have surfaces disposed in aligned relation that are respectively engaged simultaneously by said actuator to be moved in unison by said actuator when said actuator is moved from said unactuated position thereof to said actuated position thereof whereby actuation of said actuator will not cause said switch blades to contact each other unless said latch means holds said one cam in its said one position thereof when said actuator subsequently moves back from said actuated position thereof to said unactuated position thereof.
 10. The combination as set forth in claim 9 wherein the switch blade operated by said one cam has a cut-out therethrough through which said other cam projects to engage its respective switch blade. 